Polymers based on conjugated dienes and copolymers based on said conjugated dienes and vinyl aromatic monomers are known elastomers. A selected group thereof is the group composed of thermoplastic elastomers, i.e. block copolymers comprising elastomeric polymer blocks and separate polymer blocks providing physical cross-links. It is also known that the properties of these polymers (in terms of stability and aging and the like) may be improved by hydrogenating the residual olefinic unsaturation. Such residual olefinic unsaturation is the result of polymerizing the conjugated diene in 1,4-manner—leading to a C═C double bond within the polymer chain (hereinafter referred to as 1,4 double bonds)— or in a 1,2-respectively 3,4-manner—leading to a C═C bond attached to the polymer backbone as (substituted) vinyl groups.
The first hydrogenation processes appeared to hydrogenate all residual unsaturation and indeed even all aromatic unsaturation in case of aromatic copolymers. Since this had a substantial impact on the properties of the polymers, so-called selective hydrogenation catalysts were developed. These catalysts only hydrogenate the olefinic unsaturated carbon-carbon bonds. Such hydrogenation processes are nearly always based on nickel-, cobalt-, or titanium-containing catalysts.
Hydrogenating poly(1,4-butadiene) would result in a fully saturated polyolefin without elastomeric properties. It has therefore been recognized that elastomers should have a polymer backbone containing a minimum of about 20% vinyl groups (based on the content of polymerized conjugated dienes).
For certain applications, the use of selective, partially hydrogenated elastomers would be desirable. For instance, in WO 02/44281 bituminous compositions are disclosed with reduced gelation tendency. Ideally, elastomers should have a vinyl content of 20 to 65% before hydrogenation, and of less than 5% after hydrogenation, whereas the content of conjugated dienes polymerized in a 1,4-fashion of 80 to 35% before hydrogenation is not reduced to less than 30% after hydrogenation. Preferably, an elastomer having a vinyl content of 30 to 60% is hydrogenated to an extent wherein the vinyl content is reduced to less than 3%, whereas the 1,4-double bonds content is still maintained at a level of at least 30%. In the context of the present invention, the expressions “selective” and “partial” hence refer to the hydrogenation of only certain of the olefinic unsaturated carbon-carbon bonds.
Inexpensive catalysts for the partial hydrogenation of block copolymers were disclosed, inter alia in U.S. Pat. No. 3,700,748. In this document, the block copolymers were prepared by block polymerizing a monovinyl arene with butadiene, the butadiene block having a vinyl content between about 8 and 80% and thereafter were selectively hydrogenated so as to substantially eliminate the unsaturation in the pendant vinyl groups. Hydrogenation was therein carried out in a solution of an inert hydrocarbon, preferably the same hydrocarbon solvent as employed during polymerization, and in the presence of certain catalysts, e.g. nickel acetyl acetonate reduced with triethyl aluminum. According to this patent the catalyst predominantly hydrogenates all the pendant vinyl groups and not more than about 50% of the unsaturation in the 1,4-structured portions of the block. Although the patent provides an example with partial hydrogenation (iodine number of 81 corresponding to 75% conversion of the initial unsaturated olefinic carbon-carbon bonds) the selectivity has not been shown. When using nickel octanoate in a manner as described in the reference, the selectivity was found to be insufficient. Rather, on repeating the experiment of said document using a polymer having a vinyl content of 45% and using nickel octanoate at 1 ppm level, we found that at 75% conversion of the carbon-carbon olefinic bonds, the 1,4-double bonds content was reduced to less than 30%. When reducing the vinyl content to from 45% to 2%, more than 60% of the 1,4-double bonds were also hydrogenated. For polymers having initial vinyl content greater than 45%, longer times for hydrogenating the vinyl groups are required and as a result also more of the 1,4-double bonds are hydrogenated. It is clear that the hydrogenation catalyst as illustrated in this document does not provide the desired selectivity, at least not for the whole range of polymers having a vinyl content of 8 to 80%.
In a generic manner, apart from nickel, also corresponding cobalt and iron-containing catalysts were mentioned in said reference. However, the reference does not give any indication or suggestion that the corresponding cobalt or iron-containing catalysts would perform better in this respect. Indeed, using these catalysts at the same conditions as mentioned above would only have confirmed the impression that the inventor of U.S. Pat. No. 3,700,748 was too optimistic. It was found that the same polymer hydrogenated in the presence of a cobalt catalyst reduced the 1,4-double bonds content in 1 hour to less than 10%, whereas hydrogenation the same polymer in the presence of 1 ppm of an iron catalyst did not show any significant activity at all.
U.S. Pat. No. 5,705,571 discloses selective hydrogenation of conjugated diene polymers in the presence of a specific hydrogenation catalyst. Selectivity towards vinyl bonds conversion have been noted by the inventors. However, the selectivity achieved in the patent for a nickel based catalyst is comparable to that set out in the comparative example of the present patent specification.
U.S. Pat. No. 3,673,281 discloses a process for the catalytic hydrogenation op polymers containing double bonds with a iron, cobalt or nickel based catalyst in the presence of an activator. Polymers containing a vinyl content of 10% were hydrogenated at 25° C. using a relatively low hydrogen pressure of 3.5 atmospheres. The patent shows that for this set of conditions vinyl bonds are preferentially hydrogenated by a cobalt based catalyst. Furthermore, the inventors observed that an iron based catalyst is generally less active when compared with a cobalt or nickel based catalyst. However, this reference does not give any indication or suggestion that an iron based catalyst would hydrogenate vinyl bonds more selectively and/or that desirable selective, partially hydrogenated elastomers could be made using an elastomer having a vinyl content of 20 to 65% before hydrogenation.
This problem of poor selectivity was addressed and solved in WO 02/34799, disclosing methods for partially and selectively hydrogenating block polymers with other catalysts. For instance, a process for preparing a partially hydrogenated butadiene polymer was described wherein a butadiene polymer comprising vinyl groups and 1,4-butadiene recurring units was hydrogenated in the presence of hydrogen, using instead of the nickel catalyst of U.S. Pat. No. 3,700,748, a titanium-, zirconium-, and/or hafnium-based metallocene compound as hydrogenation catalyst and an alkali metal hydride co-catalyst. According to this method the vinyl bonds were converted predominantly with hardly any conversion of the 1,4-double bonds. A drawback of this method, however, is the need of very expensive metallocene catalysts. Products made by this method are therefore inherently expensive.
It is therefore an object of the present invention to provide in a cheap catalyst that nevertheless gives excellent selectivity. Surprisingly, it was now found that despite the teaching of U.S. Pat. No. 3,700,748 iron-containing catalysts, at the right conditions, provide a much better selectivity than could be expected in view of the corresponding nickel catalysts.